1. Field of the Invention
The present invention is directed generally to an improved flow manifold for analytical equipment which analyzes flowing liquid or gas, where avoidance of contamination of the sample flow from flow control valves, outgassing and valve path deadlegs upstream of the measurement cell are desired. The present invention is more specifically directed to an improved flow manifold for high purity moisture analyzers for ultra-high purity gases.
2. Description of the Prior Art
The performance of moisture analyzers developed for the analysis of high purity gases is significantly affected by quality of the surfaces in all sample wetted parts. Not only do all sample wetted surfaces have to be dried, prior to operation of the analyzer, but the surfaces must also have low affinity for sorbing water from the sample stream. While these characteristics are well known to those skilled in the art of moisture analyzer technology, surface effects of the sample wetted parts have become a major issue in the development of moisture analyzers designed to measure water vapor concentrations in the low parts-per-billion by volume (ppbv) range.
One of the most common components of any sample analysis system is a valve, and it is extremely difficult to find valves that can be easily dried adequately for use in a ppbv moisture detection system. Further, valves are a source of moisture retention and subsequent outgassing. Outgassing is defined as the emission of sorbed or occluded materials or gases. In the sample analysis system of a conventional moisture analyzer (hereafter referred to as a flow manifold) sample flow control valving is located upstream (prior to) a measuring device which is generally a measurement cell. These sample flow control valves are used to alternate the flow of sample gas, reference gas, and calibration gas into the measurement device. Valving that is located upstream of the measurement cell can have a profound impact on instrument performance, due to outgassing. In many cases the level of moisture outgassing observed from a sample valve is much greater than the moisture concentration in the sample gas that is being analyzed. In addition to outgassing, most sample systems that incorporate valves upstream of the measurement cell create "deadlegs" when the sample flow is closed off. Deadlegs are areas in the sample flow where the flow of the sample is retarded for a period of time, as when flow control valves are closed. Deadlegs are to be avoided as they provide a source of moisture retention and subsequent outgassing. Further, deadlegs that are exposed to a moisture laden sample will provide a source of moisture buildup that will be released as a surge upon resumption of the sample flow, seriously compromising the accuracy of any measurement of the moisture content of the sample.
Thus, a need exists in the art for moisture analyzers designed to measure moisture in the low ppbv range which can minimize the effects caused by valves and deadlegs located upstream of the measurement cell.